A thin film transistor (TFT) formed by the use of a thin film on an insulating surface is widely applied to an integrated circuit and the like and is used as a switching device in many cases. A display panel using the TFT has been widely used especially for a large size display device and hence there has been a growing demand for the higher definition, higher aperture ratio, higher reliability, and upsizing of a screen size.
Among methods for making a wiring in a thin film transistor like this is a method for forming a film of a conductive layer on the whole area of a substrate and then etching the film by using a mask (see patent document 1).    [Patent Document 1] JP-A-2002-359246
In the case of forming a wiring in the manner described in the patent document 1, taking an ICP (Inductively Coupled Plasma) etching unit as an example, there are cases where in accordance with etching conditions such as bias power flux density, ICP power flux density, pressure, the total flow rate of an etching gas, the rate of addition of oxygen, and the temperature of a lower electrode, a selective ratio between a resist and a conductive layer is varied to vary the width and length of the conductive layer in the substrate. Further, in the case of performing an etching process, a step of making a mask by the use of a photoresist or the like is required to elongate a process. Still further, since the conductive layer is once formed over the whole area and then is etched in such a way as to form a desired shape, wasted material is generated. These problems become more serious in the case of forming a wiring on a large size substrate having a side exceeding at least 1 m.
In contrast to this, recently, a start has already been made at studying a method for patterning a substrate directly by a liquid drop discharge method capable of forming a predetermined pattern by discharging a liquid drop containing a composition from a small hole. In this regard, a method for forming a wiring or an electrode pattern directly on a substrate with a material, in which, for example, ultra-fine metal particles are suspended in a solution, and the like have been studied. Further, in place of patterning by the use of a mask just as a conventional photolithography method, a method for forming a pattern directly by the use of a resist by a liquid drop discharge method has been also studied.
However, in the case of discharging these liquid drops by the liquid drop discharge method, a small fluctuation in a direction of discharge of the liquid drop causes a large error in the position where the liquid drop adheres. For this reason, even if the amount of discharge of the liquid drop itself is reduced, a limit is brought to the accuracy of the pattern. Moreover, if the amount of liquid drops is reduced excessively, there is presented not only a problem of reducing throughput but also a problem of reducing also the very accuracy of adhesion of the liquid drop reversely.
In the case of drawing a pattern directly by discharging the liquid drops by the liquid drop discharge method, factors causing a drawing error include an error in adhesion position caused by a small fluctuation in the direction of discharge of the liquid drop, an error caused by the resistance of air while the liquid drop is flying, and an error caused by the movement or spread of the liquid drop after adhesion. As to the first two errors, even if the manufacturing accuracy of the head is improved as much as possible, it is impossible in principle to obtain accuracy higher than probability fluctuation. In FIG. 6 is shown an error caused when a liquid drop discharged from the head adheres to the surface of a substrate. Here, it is assumed that the distance between the head and the surface of the substrate is 500 μm. Assuming that the error angle of a liquid drop discharged from the nozzle is θ, an error of adhesion position caused by this is expressed by about ±500 μm×θ. Hence, even if θ is as extremely small as 1°, the error of position becomes as large as ±8.7 μm. In addition, to this error are added an error caused by fluctuation in air flow and the like and an error caused by the spread and movement of the liquid drop after adhesion.
These problems have narrowed the application range of direct patterning by the liquid drop discharge method.
The invention has been made in view of these problems and drastically improves the accuracy of adhesion position of the liquid drop discharged by the liquid drop discharge method, thereby making it possible to form a fine and highly accurate pattern directly on a substrate. Therefore, one object of the invention is to provide a method for manufacturing a wiring, a conductive layer and a display device that can respond to upsizing of a substrate. Moreover, another object of the invention is to provide a method for manufacturing a wiring, a conductive layer and a display device that can improve throughput and the efficiency of use of material.